4 Revista Brasileira de Gcociências, Volume 22, Table 4 - Representative analyses of opaques. Symbols and letters as in table 2. Crystalhchemical formula on 4 (O) basis; Fe2O3 recalculated on ulvospinel basis Tabela 4 - Análises representativas de opacos. Símbolos e letras como na tabela 2. Fórmula cristaloquímica na base de 4 (O); Fe2O3 e FeO recalculados na base de ulvoespinélio Olivine and clinopyroxene microlites from the glassy patches have mg of 0.88 and 0.95, respectively. Mass balance calculations for the xenoliths reveal that they are composed (ΣR2 = ) of 56.7% of peridotitic assemblage (55.8% Ol Sp) and of 43.7% of intergranular material (21.9% Ol, 11.5% Cpx and 10.3% glass). Assuming that the infilled material is representative to some extent of a primary liquid, the calculated liquid (21.9% Ol, 11.5% Cpx, 10.3% glass) has mg of 0.76, corresponding, in equilibrium conditions, to an olivine with mg of 0.91, this being a value similar to that of the peridotitic assemblage. But, in this case, the olivine microlites are not in equilibrium with such a liquid, and it is believed that they represent the product of a mor evolved liquid, having presumably Cpx as the first crystallizing phase. Moreover, die mass balance (2R2>>1) shows clearly that the host nephelinites cannot be derived from the calculated liquid. On the whole, the mantle xenoliths from Estância Guavira-y are quite different respect to the Asuncion nephelinitex, which show Ol-Opx-Cpx-Sp assemblages grading from dunites to Iherzolites (Demarchi et al 1986). Estância Ramirez The sampled rocks are nephelinites with mg in the range of Texture is porphyritic with clinopyroxene (mg = 0.85), olivine (mg = 0.74) and Timagnetite (59.5 mole % ulvospinel) microphenocrysts set in a hypohyaline groundmass consisting of otivine (mg = 0.70), opaques (57.8 mole % ulvospinel) and foids (15% Q, 73% Ne, 12% Kp, in wt) microlites. The relatively low mg value in addition to the low Cr and Ni contents (cf. Tab. 1) indicate that Estância Ramirez nephelinitres represent a derivative magma. Mass balance (major elements) calculations show the impossibility of deriving this rock-type from the Estância Guavira-y nephelinites through fractional crystallization (RR2 = 0.71), a process requiring the extraction of olivine (6%), clinopyroxene (27%), nepheline (17%), titanian-magnetite (6%) and apatite (1%). Additionally, the incompatible trace element calculated/observed ratios (Rayleigh fractionation; partition coefficients after Bristow 1984) display a random range between 0.6 and 2.4, suggesting the existence of an independent parental magma. Thus, geochemical evidence points to distinctive forerunners for the San Juan Bautista nephelinites. Moreover, the mineral chemistry exhibits different features respecto to the Asuncion nephelinites. As a matter of fact, if comparison is made on the basis of major and trace elements, the San Juan Bautista nephelinites are characterized by lower SiO 2, A1 2 O 3, P 2 O 5, Cr, Ni, Ba, Nb and higher TiO 2 FeOt and Rb contents (Fig. 3). Cerro Caá Jhovy The peralkaline phonolite is porphyritic with alkali feldspar phenocrysts (44% Or, 55% Ab, in wt) and opaques microphenocrysts (71 mole % ulvospinel) set in a glassy matrix of clinopyroxene (10% acmitic component) and feldspar microlites (45% Or, 54% Ab, in wt). Five chemical analyses of samples collected at different levels of the plug (P. Comin- Chiaramonti, unpublished data) indicate that the outcrop is very homogeneous, die variations falling within the limits of the analytical error. The K 2 O/Na 2 O (wt) ratio is 0.84 and lies in the main range for the K-alkaline rock-types of the Asunción-Sapucai graben ( : Comin-Chiaramonti et al. 1992). Main differences with respect to these rocks deal with the lower A1 2 O 3, Sr and Zr and higher TiO 2, K 2 O and Rb contents of the Cerro Caá Jhovy phonolite (Fig. 4). Sr and Nd isotopes Preliminary data show that "Sr/ "Sr ratios for the host nephelinite and mantle xenoliths from Estância Guavira-y are ± 3 and ± 2, respectively (Tab. 1). Regarding the latter the isotopic ratio probably reflects the contribution of the infilled glassy material, indicating that the source for both host nephelinite and infilled material is isotopically the same. Quite different is the isotopic ratio for the Estância Ramirez outcrop, 87 Sr/ 86 Sr= ± 4. On the other hand, the CerroCaa Jhovy plug displays a very high 87 Sr/ 86 Sr ratio of ± 2.

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